1. Field of the Invention
The present invention relates to a light source for illuminating a display element and to a method for operating the same, and also relates to a display device or a display apparatus for performing display by illuminating a display element by the light source.
2. Description of Related Art
With respect to a technique for reducing the size of a projection liquid crystal display device which performs display by enlarging and projecting an image of a liquid crystal display element, Japanese Unexamined Patent Application Publication No. 51-119243 can be referred to. This publication discloses the structure of a display device in which a liquid crystal display element is illuminated by a planar light source, such as an electroluminescent element (hereinafter referred to as “EL element”), and an image displayed in the liquid crystal display element is enlarged and projected onto a screen by a lens.
Recently, with the development of organic EL elements having organic thin films as luminescent layers, the luminance is significantly increasing. An organic EL element can be an effective light source for constructing a small projection liquid crystal display device with increased brightness.
However, when the organic EL element is continuously operated with increased luminance, the luminance becomes greatly decreased. One of the reasons for this is believed to be that heat is generated by a current supplied for operating the organic EL element, and the accumulated heat increases the temperature of the element, resulting in changes in the structure and characteristics of the organic thin film.
On the other hand, as a conventional technique for suppressing a decrease in luminance of an organic EL element, Japanese Unexamined Patent Application Publication No. 7-230880 discloses a technique in which an organic EL element is operated in a pulsed mode.
With respect to a display device in which a liquid crystal display element is illuminated by three organic EL elements for emitting red, green, and blue light corresponding to the three primary colors and an image displayed in the liquid crystal display element is enlarged for display, each organic EL element must emit light at a luminance as high as approximately 10,000 cd/m2, and in order to correct the color balance according to a change in luminance of each color, the luminance of each organic EL element must be controlled independently.